Organic thin films (OTFs) have attracted considerable research interest due to their potential use as a replacement for other, more expensive semiconducting materials. Several organic materials, especially highly conjugated molecules that readily form molecular crystals such as pentacene and sexithiophene, have demonstrated semiconducting properties that approach those of amorphous silicon (Jackson, T. N., et. al., IEEE J. of Selected Topics in Quantum Elect., Vol. 4, No. 1, January/February 1998). Other superior characteristics, such as mechanical flexibility, availability of room or low temperature deposition processes, and compatibility with inexpensive flexible substrate materials make the organic semiconductors viable replacements for amorphous silicon, especially in low cost and large area applications (Dimitrakopoulos, et. al., Science, Vol. 283, 283, 1999). These characteristics make organic thin films suitable candidates for use in thin film transistors (TFTs) for active matrix liquid crystal displays (AMLCD), low speed logic and radio frequency applications such as active tags or smart cards.
The electrical performance of the OTFs at room temperature is believed to be dominated by the difficulty of moving charge carriers from one molecule to the next because of disorder, defects, and chemical impurities which can form trapping sites within the film (Garnier, F., et. al., J. Am. Chem. Soc., Vol. 115, 8716, 1993). It is generally accepted that increased ordering of the molecules in the OTF, especially in the first few monolayers deposited onto the substrate, will improve the mobility of charge carriers in the film.
The deposition conditions as well as the state of the substrate surface onto which the pentacene is deposited can have significant influence on the ordering and crystallinity of vapor deposited pentacene. Substrate temperature and deposition rate have been shown to affect the structure and size of the deposited pentacene crystals and the mobility of the deposited film (Dimitrakopoulos, C., et. al., J. Appl. Phys., Vol. 80, 2501, 1996; Gundlach, et. al, IEEE Electron Device Lett., Vol 18, No. 3, March 1997). Surface roughness has been shown to adversely affect the size of pentacene crystals (H. Klauk, et. al., IEEE Trans on Electron Devices, Vol. 46, No. 6., June 1999).
A technique that has been reported in the literature to achieve improved pentacene deposition and electrical performance is the use of self-assembled monolayers (SAMs) of alkyl silanes (such as octadecyltrichlorosilane) on silicon dioxide or glass surfaces deposited either from solution or vapor phase (Lin, Y-Y, et. al., IEEE Electron Device Letters, Vol. 18, No. 12, December 1997; Gundlach, D. J., et.al., Tech. Dig. Intl. Devices Meeting, 1999; Kane, M. G., et. al., IEEE Electron Device Letters, Vol. 21, No. 11, November 2000) or alkyl thiols such as hexadecanethiol (Dimitrakopoulos, C., et. al., U.S. Pat. No. 6,335,539B1) or 2-mercapto 5-nitrobenzimidazole (Wang, J., et. al., 41st Electronic Materials Conf., June 1999) on metal contact surfaces such as gold. Other self-assembled monolayers have not been investigated in the literature to further understand or elaborate on the influence that each monolayer has on the orientation and ordering of the pentacene film during deposition.